Questions: Cell Cycle Checkpoints and Cancer Prevention

Question 1

A tumor cell has lost p53 function due to mutation. Which consequence most directly explains why this accelerates cancer progression?

• A. The cell permanently exits the cell cycle and becomes senescent
• B. DNA-damaged cells bypass G1/S arrest and continue replicating with accumulated mutations
• C. CDK1 is constitutively activated, causing premature mitotic entry
• D. The cell can no longer produce ATP, triggering metabolic cell death

Answer: (B)

p53 normally induces p21 (a CDK inhibitor) in response to DNA damage, halting the cell at G1/S. Without p53, damaged cells skip this arrest and replicate, passing mutations to daughter cells — the engine of genomic instability. Senescence (A) requires p53 signaling to initiate; CDK1 (C) is the G2/M target, not p53's direct effector at G1/S.

Question 2

A pharmacologist develops a drug that blocks Cdc25 phosphatase activity. At which checkpoint would this drug have the most direct effect?

• A. G1/S checkpoint, by preventing Rb phosphorylation
• B. Spindle assembly checkpoint, by stabilizing the mitotic checkpoint complex
• C. G2/M checkpoint, by keeping CDK1 inactive and blocking mitotic entry
• D. Intra-S checkpoint, by stalling replication fork progression

Answer: (C)

Cdc25 removes inhibitory phosphates from CDK1, activating cyclin B–CDK1 and triggering mitotic entry. Blocking Cdc25 keeps CDK1 inhibited, arresting cells at G2/M. The G1/S checkpoint operates through Rb, E2F, and CDK2 — not Cdc25.

Question 3

p53 directly phosphorylates cyclin-CDK complexes to halt cell cycle progression after DNA damage.

• T. True
• F. False

Answer: False

p53 is a transcription factor, not a kinase. It halts the cycle indirectly by inducing transcription of p21 (CDKN1A), which then binds and inhibits cyclin-CDK complexes. The upstream kinases ATM and ATR phosphorylate and stabilize p53, but p53 itself acts through gene expression — not direct phosphorylation of cell cycle proteins.

Question 4

A cell with intact p53 signaling may respond to severe, irreparable DNA damage by initiating apoptosis rather than cell cycle arrest.

• T. True
• F. False

Answer: True

p53 can drive two outcomes depending on damage severity: temporary arrest (via p21) when repair is feasible, or apoptosis (via Bax, PUMA, NOXA) when damage is too extensive. This dual role — pause or eliminate — is central to tumor suppression. Cancer cells that disable p53 escape both outcomes.

Question 5

Why does cancer development typically require the accumulation of multiple independent mutations in checkpoint genes rather than a single mutation being sufficient?

Model answer: A single checkpoint failure is usually compensated by redundant surveillance layers — other checkpoints, DNA repair pathways, or apoptotic triggers can still eliminate damaged cells. Sequential mutations disable these overlapping defenses one by one until a cell can divide unchecked despite genomic damage.

Knudson's multi-hit model reflects layered redundancy: disabling p53 alone still leaves Rb, spindle checkpoints, and apoptosis intact. Each additional hit removes another brake. This stepwise progression also explains the steep age-dependence of cancer incidence — more time means more independent mutations can accumulate in the same lineage.